This invention relates to the operation of a fluidized-solid bed ("fluid-bed") reactor in which an exothermic chemical reaction occurs at elevated temperature and pressure More particularly this invention relates to a process for monitoring whether a fluid-bed supported catalyst with a peculiar proclivity to "stickiness" is performing satisfactorily in a reactor sought to be operated at optimum conditions, namely those at which maximum conversion to desired products is obtained at minimum cost.
"Stickiness" or "tackiness" which is not necessarily viscosity, as conventionally defined, is attributed to the degree of particle-to-particle agglomeration, viscosity or resistance to movement or separation of constituent particles. Stickiness of a supported catalyst is dependent upon the pressure and temperature of the catalytic reaction, the adsorptive quality of the catalyst, the amount and distribution of active ingredient on the surface, the number of active sites available on the catalyst and the manner and degree of their utilization, and the quantity and physico-chemical properties of the reactants and reaction products in the fluid-bed. The problems associated with operating a fluid-bed with catalysts having such characteristics are discussed in U.S. Pat. No. 4,226,798 which is incorporated by reference thereto as if fully set forth herein.
Catalyst particles which exhibit a tendency to stick to one another include supports on which are deposited "soft" elements of Group I, V, VI and VIII of the Periodic Table, and compounds thereof Most susceptible to a change in consistency are supported catalysts on which are deposited compounds of copper, iron, bismuth, antimony and the like and which additionally may be promoted by the rare earth elements and elements of Groups II, IV and VII.
To obtain a good indication of what is widely believed to be the precise condition of an operating fluid-bed, it is conventional to monitor the bed height, corresponding bed density and pressure drop continuously across the bed, and to record an hourly (or half-hourly) moving average bed height, bed density and pressure drop, and thus estimate the efficiency of the overall reaction. By "efficiency" I refer to conversion of one or more feed components to desirable products at minimum cost.
The method of selecting operation of a fluid-bed reactor and the apparatus for doing so in the '798 patent resulted from recognition of the inadequacy of prior art methods to warn of an impending "upset" whatever its cause, and particularly because measurements of bed height and pressure drop provided inadequate information of equivocal probative value, too late to avoid reaching the "point-of-no-return" or the "inversion point" of the fluid-bed. It is at this point that a peak viscosity is reached which is higher than the usual viscosity at which optimum operation of the bed is achieved.
As pointed out in the '798 patent, time is important because in an operating fluid-bed having a quantum of stickiness typical for a particular catalyst, a process upset can change the stickiness quite suddenly, and if not countered quickly, will provide no alternative but to shut the reactor down. Though the '798 invention is effective to operate a reactor with excellent control, it is not particularly suited for facile and routine use by a typical operator in the plant. For obvious reasons, it is not convenient to operate a laboratory reactor in which the fluid-bed is provided by a slipstream from the plant reactor. Even if the mechanical problems of introducing the slipstream into the laboratory reactor, and operating the torsional pendulum are disregarded, it is evident that the precise viscosity of the slipstream, after it is transferred from the plant reactor to the laboratory reactor are affected to some degree. It is more advantageous to make the viscosity measurements in situ, while the plant reactor is operating, except that methods for doing so, to date have proven inadequate.
The size of bubbles in a fluid-bed have been measured to determine temperature, as a function of time, at different points between which a temperature gradient exists, using a minute thermocouple. See "Measurement of Temperature in Bubble and Emulsion Phases" by Yamazaki, M. et al Kagaku Kogaku Rombunshu 3(3), 261(5). But this work was not related to exothermic reactions which take place in the emulsion phase since a bed operating with the reaction in the emulsion phase should show no measurable change in temperature, because the mixture is perfect and the bubbles are extremely small.
In accordance with the two-phase model of aggregative fluidization in which a fluid bed consists of an emulsion phase and a bubble phase, the emulsion phase consists of a uniform continuum of particles through which the fluid velocity is equal to the minimum fluidizing velocity, and the voidage is the minimum voidage; and, each particle is considered as free of contact from adjacent particles. Presumably, no entrainment occurs at incipient fluidization.
The point of incipient fluidization is generally defined as the lowest superficial fluid velocity at which the pressure drop across the bed (at its loosest packed density), multiplied by the total area of the bed, equals the weight of the bed charge. At this point a slight increase of fluid velocity should cause an incremental lifting or expansion of the bed and create the dense "fluid" state in which the bed particles rest more upon a cushion of gas (the fluid) than directly upon each other. A change in viscosity of the bed will affect the bed height. Prior to fluidization, the particles present a conventional fixed-bed configuration.
In another study, the static pressure distribution in the fluid-bed was measured and the behavior of the bubbles investigated by use of a hot-wire anemometer (probe). See "Behavior of Bubbles and Circulating Flow of the Emulsion Phase in a 60 cm diameter Fluidized Catalyst Bed" by Tsutsui, Toshio et al Kagaku, supra, 6(5), 501-7. But the flow of bubbles in an operating reactor is of little interest in this invention, since such flow does not determine optimum operating conditions for the bed.
In still another study it was recognized that the temperature within the larger bubbles in a fluid-bed was lower than that in the emulsion phase. But the significance of such a temperature difference at any predetermined location within the bed could only be realized if one was confronted with a catalyst having the peculiar property of "a proclivity to stickiness", and one was told that the temperature at that location would actually fluctuate meaningfully within a narrow range less than 1.degree. F. Since operation of the fluid-bed was deemed to be isothermal, there was neither reason to expect such a fluctuation nor to attribute any particular significance to such a slight fluctuation if it did exist The experimental error in the measurement of temperature with a thermocouple often exceeds 0.5.degree. F. This invention derives from having (i) used a thin thermocouple, that is, smaller in diameter than that of bubbles which will presage stickiness, and (ii) having been able to measure the slight fluctuations in temperature and identifying their significance relative to the quality of fluidization.